1. Field of The Invention
The invention relates to a flexible printed circuit (FPC) assembly, and particularly to an FPC with a ground plane and the corresponding required impedance thereof.
2. The Related Art
Historically, cable assemblies and connectors within a computer, transfer data at low speeds, which did not readily lead to signal degradation due to electromagnetic phenomenon such as impedance, skew, propagation delay, and crosstalk. As signal speed continue to increase, however, these phenomena can create severe problems within the computer system. An effort is therefore being made to control various electrical characteristics of cable assemblies and associated connector within the computer enclosure. Under this situation, the cable assembly generally utilizes a built-in grounding plane to control impedance and minimize skew, propagation delay and crosstalk.
It is also noted that in some situations two different pitch/dimension connectors are designed to be positioned at two ends of the cable assembly to respectively connect to two different pitch/dimension complementary connectors. Under this condition, for the traditional flat cable assembly, a cable having the same pitch arrangement with one connector is used and an additional small printed circuit board is used to be interface between the cable and the other connector to compensate the difference of the pitch arrangements therebetween. Alternatively, an FPC (Flexible Printed Circuit) is another approach to connect the two connectors by means that the circuit traces thereof can be designedly and intentionally configured in either a divergent or a convergent manner to meet the pitch arrangement of the corresponding connectors at either end of the FPC.
Accordingly, FPCs are used in many electronics applications where the size and the flexibility are concerns. Usually the FPC replaces several other components thus justifying the relatively high cost of this technology. In most cable assembly applications, the cost of FPCs can not be justified when compared to the standard flat ribbon cable. In addition, the performance advantage of FPC does not make much difference at slower signal speeds. However, in such a case where a transition of pitch arrangement between the end connectors occurs and the requirement for the faster data transfer rates exists, the FPC solution exhibits a very high level of performance and reliability without severe cost differential.
Understandably, even though the traditional FPC can satisfy the pitch transition between two different pitch arrangement connectors, yet the traditional FPC can not fit for the aforementioned current high speed transmission condition. As mentioned before, a grounding plane is required in the cable assembly for the high speed transmission.
Although the existing FPC (cable) assembly may include two integral layers of which one is for signal transmission and the other one is for grounding paths, the grounding layer(plane) does not provide the required impedance of the whole assembly. This is because the grounding layer and the signal layer should be spaced from each other at a significant distance by an intermediate dielectric layer. The character of the traditional FPC, which may present more than one integral signal/grounding layers therein for flexibility of disposition and miniaturization of dimension in comparison with the traditional ribbon flat cable, can not reach this requirement, anyhow.
Therefore, an object of the invention is to provide a modified FPC (cable) assembly which not only maintains the advantages of the traditional FPC cable assembly, e.g. the flexibility of disposition and the reduced dimension thereof in comparison with the ribbon flat cable, but also owns the required impedance thereof for use in the current high speed transmission.